1. Field of the Invention
The present invention generally relates to optical lenses, and more particularly to a Cooke triplet projection lens designed for an overhead projector, the lens comprising two outer positive elements of an ophthalmic crown glass and one inner negative element of a light flint glass. The lens is constructed to provide superior performance at a low cost by selection of glass materials having specific optical characteristics.
2. Description of the Prior Art
The Cooke triplet lens was first invented in 1893 (see British Patent Nos. 15,107 and 22,607, and U.S. Pat. Nos. 540,122 and 568,052) and since that time, many variations on the design have been made. Of the several uses found for triplets, photographic and projection objectives are among the most important. Photography was the first application of this lens form, and continues to be important (see U.S. Pat. Nos. 1,035,408, 1,073,789, 1,616,765, 1,658,365, 1,880,393, 1,892,162, 1,987,878, 2,064,550, 2,270,234, 2,279,372, 2,298,090, 2,388,869, 2,391,114, 2,416,033, 2,430,550, 2,582,362, 2,645,157, 2,736,234, 2,962,930, 3,194,116, 3,359,057, 3,438,696, 3,443,863, 3,449,041, 3,578,847, 3,640,606, 3,649,103, 3,784,287, 3,912,379, 3,944,337, 3,967,884, 4,542,961, British Patents 4,714 (1911), 422,246 (1933), 532,950 (1939), 601,649 (1948), 612,757 (1948), German Patent 434,759 (1924), and French Patent 1,037,274 (1953)). Among projection applications, the Cooke triplet has been used in CRT television projectors (U.S. Pat. No. 4,163,604), motion picture projectors (U.S. Pat. Nos. 2,503,751, 2,720,814,), slide projectors (U.S. Pat. Nos. 1,937,168, 3,237,520, 3,443,864 and 3,905,686) and overhead projectors (U.S. Pat. No. 3,936,155). A few patents exist for triplets used for photocopying and other applications (U.S. Pat. Nos. 1,485,929, 1,937,168, 3,202,051 and 3,584,936).
The Cooke triplet has taken two basic forms in overhead projectors: fixed focus (see the '155 patent) and varifocal (see German Patent No. 4,118,146). In the latter, the separation between lens elements is varied to adjust the focal length of the lens. The varifocal lens reduces the complexity of the projector, while the fixed focus lens is generally capable of superior performance.
A Cooke triplet lens generally consists of three pieces of glass or polymers, called elements, housed in a mechanical structure which is called the barrel. The outside first and last elements are usually of positive optical power, and the inner element is usually negative. The opposite arrangement has been tried, but found to be less desirable. Design methods for this type of lens are widely published (see, e.g., Warren J. Smith, "Modern Optical Engineering," section 12.6, or Rudolf Kingslake, "Lens Design Fundamentals," chapter 13, section V).
It is often recommended that the refractive indices of the chosen glasses be high and this practice has been maintained since the original patents. An extreme example of this is U.S. Pat. No. 3,838,910, in which the lens is constructed of glasses with a refractive index greater than 1.9. The rationale for this practice is based on the relationship between refractive index and the spherical aberration of a simple lens (i.e., single element). Increasing the refractive index of the element reduces the curvatures of the lens for a given focal length, which reduces the change in angle of incidence with pupil height, and therefore reduces spherical aberration. This suggests that high refractive indices are most helpful in lenses that work at a fast f-number.
U.S. Pat. No. 2,731,884 teaches that the average of the refractive indices of the positive elements of a triplet should be greater than the refractive index of the negative element, to improve the field coverage by decreasing the Petzval curvature of the lens. This can be understood by noting that the Petzval curvature is equal to the sum of the powers of the elements divided by their refractive indices. For the lens to have a positive focal length, the powers of the positive elements must be greater than the power of the negative element. Hence, Petzval curvature would be significant if the refractive indices were equal. Increasing the refractive indices of the positive elements with respect to the refractive index of the negative element reduces field curvature.
Overall length of the lens (also known as barrel length) is another important parameter in the design of projection lenses for overhead projectors. It is defined as the axial distance from the first lens surface to the last lens surface. As noted in the '155, a shorter lens can be built with smaller lens diameters and consequently less material, thereby reducing material and other costs. This also improves the mechanical stability of the overhead projector. U.S. Pat. No. 3,762,801 also stresses the importance of a short overall length but its emphasis is on the compactness of the resulting camera.
Smith, supra, states a general principle that reducing the difference in the Abbe values of the crown and flint elements will shorten the overall length of an optimized lens and increase its field coverage at the cost of reduced aperture. This principle is important for the design of all anastigmats, not just triplets, and gives a designer a suggestion for improving a lens in any particular application. Smith cites three patented lenses (U.S. Pat. No. 2,453,260, British Patent No. 155,640 and German Patent 287,089) as examples of good designs that have a larger Abbe value difference for a fast, smaller field lens (.DELTA..nu.=22) and a smaller Abbe value difference for a slower, larger field lens (.DELTA..nu.=15). Conversely, U.S. Pat. No. 3,762,801 describes shorter, slower, narrower field lenses with similar Abbe value differences, but requires an aspheric component or high refractive indices for adequate aberration correction.
A teaching that is implicit in several patents, including U.S. Pat. Nos. 2,818,777, 3,762,801, 3,838,910, 3,910,685, 4,105,308, 4,109,995 and 4,787,724, is the use of a meniscus positive element. All of these patents describe the use of such an element, but fail to teach any advantage associated therewith. According to Kingslake (p.240), "Gauss once suggested that a telescope objective could be made with two meniscus-shaped elements, the advantage being that such a system would be free from spherochromatism." He continues by noting that Alvan Clark had the insight that two of these objectives, back to back, would make a good camera lens. That camera lens is now widely known as the Double Gauss type and is used for high quality camera objectives. Lenses of this type have a high degree of correction. Another well know use of meniscus elements is as spherical aberration "correctors" in Bouwers-Maksutov telescopes (Kingslake, p. 311ff.). In these lenses, the meniscus is used to compensate the spherical aberration of a spherical primary mirror.
In spite of the foregoing teachings, prior art Cooke triplet lenses, particularly those suited for use in overhead projectors, still suffer certain disadvantages such as spherical aberration, coma and astigmatism, and while a given one of these defects may be improved by adjusting certain parameters of the elements, the other defects are magnified in the process. There is also a constant drive in the manufacture of overhead projectors for a decrease in manufacturing cost, while maintaining or improving product quality. It would, therefore, be desirable and advantageous to devise a construction of a Cooke triplet lens which is compact and relatively inexpensive, and yet which achieves superior performance in the noted characteristics, i.e., generally provides a sharper, true image.